Fixed an accessibility issue with the pin view

We were sending an accessibility event even if there were no
characters left, leading to an additional "bullet deleted"
notice.

Test: input pin, delte pins one bye one, observe no talkback spoken anymore at the end
Bug: 62714260
Change-Id: Iee751e31f702f4b0dc6d5a15d31d58e71bfa797e
(cherry picked from commit 8a72b06a)